1. Technical Field
The invention relates to the field of electrochemical gas generators, and more particularly to ceramic oxygen generating systems (COGS) for generating and delivering oxygen to a user.
2. Background Art
There are a variety of ways to generate and deliver substantially pure oxygen for use in medical applications and in the electronic industry, for laser cutting, and in many other applications. Two key areas of use for pure oxygen are breathing purposes in home oxygen therapy (HOT) patients and for aviation crewmembers.
Oxygen can be stored in cylinders at elevated pressures and stored as liquid oxygen in what is commonly called a Dewar. These techniques are of common knowledge and have been utilized for many years.
More recently oxygen can be concentrated or generated real-time using a variety of molecular sieves to separate and concentrate oxygen from a pressurized air source using a pressure swing adsorption process (PSA) or a vacuum pressure swing adsorption (VPSA) process. Oxygen product generated by pressure swing adsorption is low cost and readily available. However, the oxygen concentration on these products is about 90 to 95%.
Even more recently oxygen can be generated using an electrochemical process that ionizes the oxygen molecules at one surface of a ceramic membrane, transports the ions through the membrane, and reforms them as oxygen molecules on the other surface of the membrane by removing the excess electrons from the ions. This type of technology is also known as a ceramic oxygen generating system (COGS).
Oxygen conserving devices that provide a short pulse of oxygen to the user through a nasal cannula at the beginning of inhalation have been widely used for many years to reduce the quantity of supplemental oxygen delivered to patients who suffer from chronic lung disorders while still maintaining an adequate oxygen saturation level in the blood stream. This has been commonly referred to as pulse dosing of oxygen.
This type of oxygen pulse dosing technology has recently been used on aircraft to provide supplemental oxygen to pilots of general aviation aircraft that typically fly at altitudes of up to 32,000 feet. Similarly, oxygen breathing regulators have been used for many years to reduce the amount of oxygen consumed by aerospace crewmembers at various altitudes to minimize the possibility of oxygen toxicity of the user, while providing a minimum level that is a physiologically safe oxygen concentration to prevent the occurrence of hypoxia at higher altitudes. Physiologically safe concentrations of oxygen have been provided by the breathing regulators that entrain aircraft cabin air and add it to the stored or generated oxygen before delivery to the crewmember to lower the concentration to the desired level as a function of atmospheric pressure-altitude. This is most commonly accomplished using the well-known pneumatic injector device. This technique has also been used to dilute the oxygen generated from PSA oxygen concentrators in aircraft to the desired concentration.
If an aircraft is equipped with the PSA oxygen concentrator and cabin pressure of an aircraft is lost above an aircraft altitude of about 30,000 feet, then the purity of the oxygen being generated by the PSA, which is only about 95% Oxygen, does limit the time of exposure that the crewmember can safely stay at the higher altitudes. However, if the aerospace oxygen system is comprised of stored oxygen of 99.5% Oxygen or has a back-up oxygen system (BOS) that contained 99.5% oxygen, then additional aircrew safety can be provided in the event of the loss of cabin pressure at these higher altitudes. However, the use of 99.5% oxygen in either the high-pressure storage or liquid form requires periodic replenishment ranging from daily to weekly periods of time and results in a logistics burden to the user.
The implementation of the PSA oxygen concentrators into aerospace applications has solved the requirement for oxygen on a daily basis, but has had limited acceptance as a supply of oxygen for use after the loss of cabin pressure at the higher altitudes. Currently, only the F-15E aircraft uses a back-up oxygen supply that is automatically filled with 93% oxygen that is generated and stored using a PSA oxygen concentrator with an integral compressor. An emerging limitation to using PSA oxygen concentrators on next generation aircraft is the trend towards the use of more efficient engines with higher bypass ratios that result in less compressed air being available for the PSA oxygen concentrator and other air handling systems on the aircraft. This results in market pressures to minimize or eliminate the use of compressed air from the aircraft engine.
Many aerospace applications need a way to generate oxygen at a purity of greater than 99.5% and store it at an elevated pressure for emergency conditions, while providing a lower concentration for normal flying conditions when the aircraft cockpit is equivalent to a pressure-altitude of less than about 25000 feet. This system needs to be compatible with current aircraft personal equipment that includes oral-nasal masks that can deliver the oxygen at pressures up to about 70 mmHg under certain normal and emergency flying conditions.
Similarly, many home oxygen therapy patients can benefit from an oxygen generator that can deliver oxygen in the home via a pulse-dosing oxygen conserving device using a nasal cannula during, while also storing oxygen in a portable cylinder for temporary use outside the home. This reduces the logistics burden of the home health care provider by eliminating the need to replenish the portable oxygen cylinders after use. Some patients that have sleeping disorders are better served by a continuous flow from the nasal cannula in order to maintain the desired oxygen blood saturation levels while they sleep.
Certain ceramic materials, when subjected to specific conditions, will actively pass oxygen atoms through its matrix. Previous embodiments of this concept have shown that a properly designed ceramic membrane can act as both a means for concentrating 100% oxygen and for pressurizing oxygen to levels suitable for recharging conventional oxygen cylinders.
A basic ceramic oxygen generating system generally consists of one or more temperature controlled ovens which contains ceramic oxygen generating elements. In order to supply enough oxygen for the ceramic elements, air is circulated inside the oven. Since the ceramic elements operate at high temperature (about 700° C.), air has to be heated before inputting to the oven. Heat exchangers are used to preserve the heat and reduce temperature of oxygen-depleted air exhausted from the oven.
Major problems of such a basic COGS described include:                The ratio of air input flow to oxygen output flow is very high (20 to 1). This high input flow requires having heat exchangers to avoid heat loss or thermal shock to the ceramic elements.        Heat exchangers efficiency needs to be high to preserve the heat. Therefore, they are expensive.        To exchange the heat, the oven has to be pressurized to generate a return (oxygen-depleted exhaust) flow. The positive pressure generates thermal leak around openings/cracks on the oven.        
Oxygen at high purity levels, high pressures, and high temperatures creates a hazardous environment because it will support vigorous combustion of many conventional materials. Thus, it is very difficult to design a cost-effective pressure vessel capable of holding pure oxygen at high pressures and high temperatures. Previous embodiments of this concept have utilized the ceramic membrane as an initial pressure vessel for the high purity oxygen, until it can be cooled enough for safe transfer to, and storage in, conventional oxygen cylinders.
Using the ceramic membrane as a pressure vessel incites a powerful conflict between maximum operating pressures and the rate of oxygen concentration. The ceramic material is relatively brittle, thereby ill suited for withstanding the tensile stresses associated with internal pressurization. Increasing the wall thickness of the module will lower the material stresses, but it will also lower the module's oxygen concentration rate and operating efficiency.
Ceramic oxygen generating modules such as the Integrated Manifold And Tubes (IMAT) described in U.S. Pat. Nos. 5,871,624; 5,985,113; 6,194,335; 6,352,624; and others have been used in oxygen generating systems. However, in most of the applications, the IMAT's outside surfaces are interfaced with air and oxygen is pumped through the ceramic to the inside cavities (circuit). This design limits the maximum oxygen generating pressure due to high tensile stresses on the ceramic.
While the above cited references introduce and disclose a number of noteworthy advances and technological improvements within the art, none completely fulfills the specific objectives achieved by this invention.